Combining multiple textures with a white background
Here's an example with two images with a white background:

To keep the white and mix the two colors you can use a Color MixRGB node with Multiply as the transfer mode. Then plug that to only one Diffuse BSDF shader:


Using Multiply as the transfer mode (quoting from the wiki)

Returns a darker result than either pixel in most cases (except one of them &gt;equals white=1.0). Completely white layers do not change the background at all. &gt;Completely black layers give a black result. The &quot;opposite&quot; to Screen mode.

More info here